Asphalt concrete is a mixture of asphalt cement and aggregate materials of various sizes. Production facilities for making asphalt concrete to be used as a paving composition are well-known, and various such facilities may allow for batch production or continuous production. U.S. Pat. No. 4,867,572 of Brock et al. describes a type of continuous production plant known as a counter-flow dryer/mixer. This machine comprises an inclined inner drum mounted for rotation about its long axis. A non-rotating outer drum is disposed around the rotating inner drum so as to form an annular mixing chamber between the outside of the inner drum and the inside of the outer drum. Paddles are mounted on both the inner and outer surfaces of the inner drum. A burner is located at the lower end of the inner drum, and aggregate materials are introduced into the upper end of the inner drum. Because of the inclination and rotation of the inner drum, as well as the location of the paddles on the outer surface of the inner drum, aggregate materials that are introduced into the upper end of the inner drum are dried and heated as they are tumbled down towards the lower end and towards the source of the burner flame.
At the lower end of the inner drum, the dried and heated aggregate materials are discharged into the annular mixing chamber between the inner drum and the outer drum. Liquid asphalt cement is also introduced through an AC port into this annular mixing chamber, and continued rotation of the inner drum causes the asphalt cement to be thoroughly mixed with the heated and dried aggregate materials to produce an asphalt concrete mixture. The paddles on the outside of the inner drum assist in this mixing as the inner drum is rotated, and they also serve to direct the asphalt concrete mixture in the annular mixing chamber towards the upper end of the inner drum to an asphalt concrete discharge outlet.
Some embodiments of the counter-flow dryer/mixer include an inlet into the inner drum for recycled asphalt product (“RAP”). In these embodiments, RAP, which is also comprised of asphalt cement and aggregate materials, may be introduced into the inner drum where it will be heated, tumbled and dried with the aggregate materials therein. The RAP must be heated sufficiently to melt the asphalt cement therein so that the components of the RAP can be thoroughly intermixed with the virgin aggregate materials and the asphalt cement in the annular mixing chamber.
Because asphalt cement is an excellent binder of aggregate materials, and because the production of asphalt concrete in a counter-flow dryer/mixer is a dynamic process, it is common for some asphalt concrete, as well as asphalt cement and mixtures of asphalt cement and fine aggregate material used in the production process, to accumulate on the outer surface of the inner drum. Since the dryer/mixer is heated to a high temperature prior to beginning the production cycle, and since it remains hot for a time after production has ceased, accumulated asphaltic material will oxidize over time to form a hard build-up. As the inner drum expands and contracts upon heating and cooling during normal use, this oxidized material will crack and break loose from the outer surface of the inner drum. This loose material falls into the annular mixing chamber where it will contaminate subsequently produced asphalt concrete. When asphalt concrete that is contaminated with oxidized build-up material is spread with a paving machine, the oxidized material may be pulled by the paver screed through the newly laid mat of asphalt concrete, thereby damaging the surface of the asphalt mat. Any damage of this type must be quickly repaired by hand prior to compaction of the mat, thus resulting in delays in completing the paving project and unnecessarily exposing workers to the hot asphalt concrete. Furthermore, when build-up becomes a problem, the asphalt dryer/mixer must be taken out of production and partially disassembled so that the build-up material can be removed from the outer surface of the inner drum.
It is known that operating the asphalt dryer/mixer at a low tonnage rate increases the likelihood that (or the rate at which) oxidized material will build up on the outer surface of the inner drum. However, increasing the tonnage rate of the asphalt dryer/mixer to reduce the rate of build-up will also increase the rate of wear of the inner and outer drums, and the rate of wear of the paddles mounted on the inner and outer surfaces of the inner drum. It is also known to modify the tips of the paddles on the outer surface of the inner drum to slow the movement of asphalt mix through the annular mixing chamber and thereby mimic a high tonnage rate. However, such a modification requires much time and hand work and also involves the expense of the modified tips added.
It would be desirable if a method and apparatus could be provided that would allow for modifying the asphalt dryer/mixer in a relatively simple and inexpensive way to minimize the build-up of asphalt cement and asphalt concrete material on the outer surface of the inner drum.
Notes on Construction
The use of the terms “a”, “an”, “the” and similar terms in the context of describing the invention are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The terms “substantially”, “generally” and other words of degree are relative modifiers intended to indicate permissible variation from the characteristic so modified. The use of such terms in describing a physical or functional characteristic of the invention is not intended to limit such characteristic to the absolute value which the term modifies, but rather to provide an approximation of the value of such physical or functional characteristic.
The steps of any method described herein in connection with the preferred embodiments of the invention can be performed in any suitable order unless otherwise indicated herein, explicitly or by context. The use of any and all examples or exemplary language (e.g., “such as”) herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. Nothing in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
The following defined terms, and any other terms specifically defined herein, are to be given their broadest possible construction consistent with such definitions:
The terms “asphalt dryer/mixer”, “dryer/mixer” and similar terms refer to an asphalt production machine that is adapted to heat and dry aggregate materials and mix such materials with asphalt cement, as well as other materials known to those having ordinary skill in the art to which the invention relates.
The terms “adapting an asphalt dryer/mixer”, “modifying an asphalt dryer/mixer”, “adapting the inner drum of a dryer/mixer”, “modifying the inner drum of a dryer/mixer” and similar terms, when used in connection with the invention, refer to the use of the invention in original equipment manufacture, as well as in retrofitting existing equipment.
The term “aggregate materials” and similar terms refer to crushed stone and other particulate materials that are used in the production of asphalt concrete, such as, for example, crushed limestone and other types of crushed stone, shredded or comminuted mineral and cellulosic fibers, gravel, sand, lime and other particulate additives.
The terms “asphalt cement”, “AC” and similar terms refer to a material that is used in combination with aggregate materials in the production of asphalt concrete. Asphalt cement acts as the binder for various aggregate materials in the production of asphalt concrete.
The terms “recycled asphalt product”, “RAP” and similar terms refer to a comminuted or crushed product containing aggregate materials bound together by asphalt cement. RAP typically comprises crushed or comminuted recycled asphalt paving materials, crushed, shredded or comminuted shingles and other asphalt cement-containing products.
The term “asphalt concrete” and similar terms refer to a bituminous paving mixture that is produced, using asphalt cement and any of various aggregate materials, in an asphalt dryer/mixer or other asphalt concrete production plant. Asphalt concrete may be made with any of various aggregate materials, asphalt cement and RAP.